1. Field of the Invention
The present invention relates to a detecting circuit for a playback signal in a magnetic recording system which uses partial response, and more particularly to a detecting circuit which detects a playback signal which is played back from a head of a magnetic disk apparatus which uses partial response.
2. Description of the Related Art
In recent years, in magnetic disk apparatuses, partial response and detecting circuits using the ML (maximum likelihood) detection method have come into practical use in detecting a read signal from a head.
FIG. 1 shows the overall configuration of a prior art magnetic disk apparatus. A plurality of annular tracks are formed on a magnetic disk 1, and data is written onto these tracks. A head 2 writes data onto these tracks, and reads out data from the tracks. This head 2 is positioned over the magnetic disk 1 by means of a servo circuit 90. The head 2 is connected to a head IC 91, which has a write circuit and an amplifier which amplifies the read-out signal. This IC 91 has connected to it a read/write circuit 92, which handles track information, zone information, and head information. The operation of the servo circuit 90 and read/write circuit 92 is controlled by a control circuit 93.
FIG. 2 shows an example of the configuration of the detecting circuit used in a magnetic disk apparatus of the past. A signal which is recorded onto the magnetic disk 1 is read out by a head 2. The thus read-out signal is passed through an amplifier 3 and a low-pass filter 4, is equalized by an equalizer 5, and is then sent to a detector 6. In the magnetic disk apparatus, partial response and the PRML systems which are coming into practical use, are configured in the same manner.
The most typical system is the PR4ML (partial response class 4) system. If the data bit period delay is expressed as D, the magnetic recording system can be treated as 1xe2x88x92D (step response of 1). Also, because 1+D is used as an equalizer, the equalized output becomes 1xe2x88x92D2, this being treatable as the class 4 partial response. In addition, a maximum likelihood detector (generally based on the Viterbi detector) is used, the 1xe2x88x92D2 signal is used to detect data.
However, strictly speaking, because of excessive inter-symbol interference, the step response in magnetic recording (an isolated waveform which is the playback by a head of a single magnetization reversal) is not 1. For this reason, an equalizer is used to remove this excessive inter-symbol interference.
It is known that the performance of PRML differs depending upon the partial response class (equalized waveform). For this reason, the inventors of the present invention have already demonstrated a method of performing optimum equalized detection, in the form of a xe2x80x9cViterbi Decoding Control Systemxe2x80x9d (Unexamined Patent Publication (Kokai) No. 4-221464) and xe2x80x9cViterbi Decoding System Including Variable-Order Equalizerxe2x80x9d (U.S. Pat. No. 5,287,385). In this system, a single equalizer switches the partial response class (the order n in (1xe2x88x92D) (1+D)n), in response to which the Viterbi detector reference (assumed value) is switched.
However, the size of the Viterbi detector circuit in a maximum likelihood detection system which also uses partial response is an exponentially increasing function of the above-described partial response order n. Therefore, in the system proposed by the inventors of the present invention, because it is necessary to design the detecting circuit for the maximum value of this order n, problems exist with regard to an increase in circuit size and power consumption.
In view of the above, an object of the present invention is to provide a detecting circuit in a magnetic recording and playback apparatus which uses partial response and the maximum likelihood detection method, the use of which is accompanied by an increase in neither the size nor the power consumption of the circuit in the magnetic recording and playback apparatus, so that the size of the apparatus is not increased even if the partial response order n increases.
To achieve the above-noted object, the present invention is applied to a magnetic recording and playback apparatus in which data, which is recorded on a magnetic recording medium by a head, is read out, this apparatus having an amplifier circuit, a low-pass filter, an equalizer, and a detector for the purpose of detecting the data from the thus read-out signal.
In the detecting circuit of the first aspect of the present invention, after the amplifier circuit and low-pass filter, a plurality of series-connected equalizers and detectors are provided, these being followed by a switching control means. The equalizers are provided for differing equalization target waveforms. The detectors are connected in series with each of the equalizers in accordance with an equalized waveform therefrom and each detector forms a set with an equalizer. These equalizer-detector sets are connected in parallel to the output of the low-pass filter. The switching control means is connected to the outputs of the detectors of the equalizer-detector sets and selects the output of one of these equalizer-detector sets as the detected data. Only the data which is selected by the switching control means is output as the detected output of the detecting circuit of the first aspect of the present invention.
The second aspect of the present invention has an equalizer disposed after an amplifier circuit and a low-pass filter, a plurality of detectors which are connected in parallel to this equalizer, and a switching control means which selects one of the detected data which is output from one of the plurality of detectors. The switching control means selects just one output of detected data from the plurality of detectors as the detected output.
The detecting circuit of the third aspect of the present invention is applied in an apparatus in which the frequency response of the signal read out by the head is expressed as R, and in which the frequency response of the equalization target waveform of order n is G(f)*Hi(f) (where i=1, 2, 3, . . . , n). Therefore, in the detecting circuit of the third aspect, the equalizers are divided into one pre-equalizer and a plurality of n post-equalizers. The pre-equalizer is configured so as to have a response of G(f)/R(f), and the post-equalizers are characterized to have responses of Hi(f) (where i=1, 2, 3, . . . , n). A detector is connected to the output of each of the post-equalizers, these forming post-equalizer-detector sets. These post-equalizer-detector sets are connected in parallel to the output of the pre-equalizer. The output of each of the post-equalizer-detector sets is connected to a switching control means, which selects just one of the detector outputs as the detector output.
In accordance with the present invention, a plurality of equalizers and detectors are provided in parallel to suit a variety of equalization target waveforms, these being switched in accordance with the equalization target waveform. Therefore, in the present invention, it is possible to limit the size of the circuit of each equalizer and detector to the minimum required for the equalization target waveform. As a result, with the present invention, even if the partial response order n increases, no increase is caused in the size of the detecting circuit.
Each of the equalizers and detectors is provided independently. For this reason, by stopping the operation of unneeded circuits which are not selected by the switching control means, it is possible to limit the increase in power consumption.
In the present invention, because it is possible to perform optimum equalization and detection with respect to the normalized recording density on the magnetic disk, it is possible to decrease the required S/N ratio. In addition, with the present invention, for a given S/N ratio it is possible to increase the recording density.